The present invention relates to video coders and, specifically, to variable length coders utilized in the coding of video images.
Currently, several standards for video coding have been defined or are under consideration. The proposed standard for video coding of motion pictures, MPEG-4, provides for coding according to a discrete cosine transform, quantization, run length coding and additionally variable length coding (VLC). VLC tables are preferred due to their low complexity. They permit bit savings by allocating shorter codes to video signals that occur with high frequency and longer codes are allocated to video signals that occur with lower frequency. However, VLCs are narrowly tailored for specific applications; they do not adapt to dynamic conditions. For example, for intra coding in the verification model no. 6 of the proposed MPEG-4 coding standard, a first VLC table is used to code DC coefficients of luminance, a second VLC table codes AC coefficients of luminance, a third VLC table codes DC coefficients of chrominance and a fourth VLC table codes AC coefficients of chrominance. For inter coding, additional VLC tables may be required. However, the addition of VLC tables increases system complexity and requires additional signaling to identify when specific VLC tables are in use. Such overhead consumes bandwidth and contributes to coding inefficiencies.
Video coders now may advantageously employ variable quantizers. Quantizers divide down a video signal by a quantization parameter to reduce bandwidth occupied by the coded signal. A variable quantizer divides down by a variable quantization parameter but it changes the statistics of the video signal coded by the VLC tables. Thus, changes in the quantization parameter affects the coding efficiencies of the VLC tables. Coding savings obtained at the quantizer may result in costs at the VLC table.
Accordingly, there is a need in the art for a coding scheme that integrates a variable quantizer with a variable length encoder. Further, there is a need in the art for such a coding scheme that switches among VLC tables without additional overhead signaling.